In optical transceivers, it is desirable to integrate as many photonic components in one chip as possible. As integration density increases and the sizes of photonic components shrink, however, it is increasingly difficult to integrate the photonic integrated circuit (PIC) with other optical components such as fibers because the mode size in the waveguide of the PIC shrinks correspondingly. For example, the mode size of a typical 450 nanometer (nm)×220 nm wave-guide in a silicon photonic based PIC is roughly the size of the waveguide itself, but the mode size (e.g., mode field diameter) of a standard single mode fiber is as large as 9.2 micrometers (μm). Thus, the mode size of the waveguide on the PIC is much smaller than the mode size of the fiber.
Lenses may be used to reduce the mode size of the fiber to correspond to the mode size of the waveguide of the PIC. However, packaging the PIC with lens and fiber is challenging due to the limited alignment tolerance if the mode size at the interface of the PIC is too small.
A mode converter may, for an edge coupled PIC, be used to enlarge the mode size of the waveguide of the PIC. However, the mode converter should be able to be integrated with other components on the PIC as well.